As used in this passage, the phrase “requires less purification” means:
People often complain about beavers building dams on their property and causing damage. Actually, these beaver dams do people a favor by serving as a filter for the water that flows through them. The water on the other side of the dam is actually cleaner than it was before it went through the dam. So, the filtered water requires less purification before people can drink it.
needs chemicals added
does not need as much cleaning
is not needed by the body
does not need to be pumped
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